For a pedant you don't half blunder on into irrelevancy! The "European Union" wasn't a public issue in his time, and he was clearly referring to the EEC.
Thanks for the "research" anyway, as I'd always assumed that (like naive young me) he would have taken what he was told at face value, and voted for EEC membership. In fact the reason I raised the matter was simply his sage advice "That is no way to go into a negotiation. You ought to go into a negotiation on the basis that they have need of you, not just you of them." Pretty relevant to Mrs May & Co. right now I would have thought, and hopefully obvious to other readers. And... that's the fullness of it - no essay required!
P.S. I sort of cringe to say it - unpedantic as is my nature - but I've always assumed the old BUDC was spot on when named Attlee Park!